The accurate measurement of ambient fluid (air) flow is becoming increasingly more important in the application and control of many processes, as well as in the research laboratory. One of the major applications is in the field of air sampling, in which an accurate knowledge of the sampled air quality determines the exposure level to various contaminants. The most widely accepted, primary standard method of flow measurement for a gaseous fluid is the bubble flow meter. In the basic form of the bubble flow meter, a soap film is generated from a soap solution, and is propelled by the gas flow under measurement from one end of the flow meter to the other. By timing the rise of the soap film between calibrated volume marks, the volume flow is obtained. Since for all practical purposes, the soap film is massless, it requires almost no force to accelerate the bubble. Furthermore, a seal is always insured by the presence of the bubble. The very nature of the bubble eliminates the friction which is associated with a piston-type flow meter. The soap film flow meter is essentially transparent to the flow being measured, having a no-load effect. Accordingly, the soap film flow meter comes closest to meeting the unique requirements of the ideal calibrator.
The measurement of air flow using a positive displacement reciprocating piston flow meter is susceptible to errors based on the following requirements:
a) Initial breakaway friction; PA1 b) Acceleration and deceleration of the piston after breakaway (until equilibrium is reached); PA1 c) Running friction; and PA1 d) Fixed pressure loading determined by the mass of the piston. PA1 (a) a hollow flowtube vertically oriented to form a top and a bottom end; PA1 (b) a piston disposed in said flowtube for movement between a bottom position, adjacent said bottom end, and an elevated position relative to said top end; PA1 (c) stop means located at said top end and said bottom end of said flow meter, respectively; PA1 (d) means for connecting said flowtube to an external pump for directing a fluid through said flow meter at a flow rate to be measured by said flow meter; PA1 (e) a valve assembly integrated in said piston for movement in concert therewith, with said valve assembly having a valve-open and valve-closed position, said valve assembly comprising: a valve body, at least one valve passageway for providing fluid access through said piston in the valve-open position, a valve head, a movable valve stem, slidable within said valve body and connected to said valve head, and latching means for holding said valve assembly in said valve-closed position until said valve stem strikes said stop means, such that upon reaching said top end, said piston is caused to reverse direction and to descend by gravity to said bottom end, and upon reaching said bottom end, is caused to reverse direction and ascend back to said top end; and PA1 (f) photoelectric sensor means arranged at different positions along said flowtube, relative to said bottom position and elevated position, for detecting the presence of said piston at each such position.
The resolution of the above condition presents a load to the air flow system being measured. The arrangement of the present invention incorporates a valve and a magnetic latch within the piston assembly which minimizes the initial breakaway friction and acceleration forces of the piston on reversing its direction at the bottom end of the piston stroke, and is accelerated downward at the top end of the piston stroke. An additional advantage of the positive displacement piston flow meter of the present invention is its simplicity in design for reversing the direction of the piston on both the upstroke and down-stroke of the piston.